Geothermal energy- an abundant sustainable power source

What is geothermal energy?

Put simply, geothermal energy is the energy stored and flowing as heat beneath the surface of the earth. This heat comes from two fundamental sources. Firstly, heat remaining from the original formation of the earth. This residual heat can be pictured most readily as the extremely hot molten outer core and solid inner core, and mantle of the earth; the heat from these gradually travels up through the thousands of kilometres of rock to the earth’s crust, where it flows through the earth’s surface. Secondly, there is heat generated locally within the earth’s crust, from the natural decay of the radiogenic elements uranium, thorium and an isotope of potassium. These occur in almost all rocks, but in certain granitic bodies, they can be concentrated such that there is a marked elevation in the local surface heat flow.

The average ‘heat flow’ through the earth’s crust is about 87 milliWatts per square metre of the surface (87mW/m²). Compared to a household radiator rated at (say) 2000 W, the average heat flow per square metre is a very small number, but when combined with the large area of the earth’s surface, the global heat flow is very large, approximately 44 teraWatts (1 teraWatt = 10¹² or a million million watts).

The amount of heat flow in the earth is not uniform. Beneath oceans, the average heat flow is about 101 mW/m²; beneath continents the value is about 65 mw/m². However, there are areas within continents where the heat flow is very much larger due to either localised igneous activity or the presence of Thermally Anomalous Granites, and where this occurs, there is the potential for the geothermal energy to be harnessed to produce electricity, or to be used directly in heating and drying applications.

How much stored energy?

As the potential to utilise the energy stored as heat in the shallow earth’s crust has become apparent, various agencies have begun to estimate the amount of energy that may be accessible.

A recent report by the Massachusetts Institute of Technology, looking at energy stored in rocks in the USA between 3 and 10km deep, estimated 13.3 million exaJoules (1EJ = 10¹⁸ Joules = 277 million megaWatt hours) of conduction-dominated ‘Enhanced Geothermal System’ (EGS) resource in crystalline basement rock formations. EGS as used in the USA means the same as EGP used by KUTh. This greatly exceeded the energy stored in other heat systems, such as volcanic and ‘hot springs’ types of areas and is 13,000 times the consumption of primary energy in the United States in 2005. Of course the economically extractable amount will be much lower. The study attempted to estimate a recoverable EGS resource and if only 2% of the total resource was recoverable, it was found that this would amount to approximately 280,000 exaJoules (78 million million MWh) or 2,800 times the 2005 US energy consumption.

A similarly detailed study has not been done for Australia, but preliminary figures from Geoscience Australia estimate that Australia’s hot rock energy between the depth corresponding to a minimum temperature of 150°C and a maximum depth of 5,000 m is approximately 1.2 million exaJoules (333 million million MWh) or 20,000 years of Australia’s primary energy use in 2005; again this is an estimated total resource figure and not an estimate of recoverable or economic energy. This resource figure is currently under review.

Producing electricity from geothermal energy

One of the most attractive uses for geothermal energy is to convert it into electricity. This was first done at Larderello, Italy in 1904 using steam from a natural geothermal field. Since that time, harnessing of steam and hot water at or close to the surface for electricity has been undertaken at many locations throughout the world, including California, New Zealand, Iceland, Indonesia, Mexico and the Philippines. The installed world generating capacity of this type of generation is nearly 10,000MWe. These projects have mostly tapped thermal resources associated with recent igneous and volcanic activity. Because of its geological setting, Australia does not present opportunities of this type.

Electricity can also be generated from geothermal waters that are not boiling at the surface. Sub surface aquifers in rocks can contain water heated from below or laterally, but perhaps only to temperatures much less than 100°C. These aquifers can be drilled into, and the heated waters pumped to the surface and the heat used to generate electricity, albeit at lower efficiency than the surface boiling waters mentioned above.

Although common around the world, the only geothermal energy currently being generated in Australia is from a small binary power station at Birdsville, Queensland, which uses hot water sourced from the Great Artesian Basin flowing at 98°C and is rated at 120kW. Although the resource has not been explored extensively, there is no real shortage in Australia of such hot artesian waters which might be utilised to generate electricity. The reason for the lack of development is probably due to the abundant alternative sources of energy and the relative low efficiency, using current technology, of producing power from lower temperature waters.

The oil price shocks of the 1970s stimulated research in the United States and elsewhere on a possible third source of geothermal energy to produce electricity. Certain granitic bodies are well known to contain higher than average concentrations of the radiogenic elements and isotopes of potassium (K⁴⁰) uranium (U) and thorium (Th) and consequently, these bodies produced heat flows well in excess of the continental average. It was reasoned that if such bodies were covered by a thickness of insulating cover rocks, trapping the heat, then there may be a large reservoir of very hot rocks accessible by drilling. If this heat reservoir was found or was made to be permeable, it could act as a heater for waters injected down one drill hole and the super heated water could be extracted from an adjacent drill hole. The very hot water would then be used to generate electricity at high efficiency, and then re-injected back down again, to be re-heated. Such a system has become known by a variety of terms, including ‘hot dry rocks’ (HDR), ‘enhanced geothermal systems’ (EGS) and ‘enhanced geothermal power’ (EGP). KUTh Energy prefers the latter term, as it describes the need to enhance the natural setting in order to produce electricity.

Enhanced Geothermal Power (EGP)

The EGP process can be summarised as follows:

Identification of an area of high heat flow, usually caused by a thermally anomalous granitic body, with the granite buried by thick sedimentary or other rock cover;
Detailed heat flow measurements and modeling to establish the deep heat reservoir as being probably hot enough to support efficient electricity generation;
Drill down into the heat reservoir; this may be through 3+km of cover rocks and 1+km into the granite (at present 5km is the limit of drilling technology);
Stimulate or enhance the permeability of fractures within the granite by pumping water at very high pressures into the granite. This is technology adapted from the oil industry;
Drill a second hole into the enhanced fracture system and demonstrate connectivity and adequate heat exchange and fluid flow between the two holes;
Drill a third hole into the fracture system to complete a circuit whereby the original hole acts as the cool water injection hole and the other two holes act as extraction holes for super heated water. The hot water is put through a heat exchanger and the heated second fluid used to drive a turbine to generate electricity. The original water, having lost most of its heat, is re-injected down the first hole to repeat the cycle. Flow directions and pressures are maintained such that little water is lost underground.

EGP was first investigated in the United States in the early 1970s at Fenton Hill, New Mexico. This site was chosen in part because of known high (approx 200°C) rock temperatures at relatively shallow depths (approx 3km). The heat reservoir was drilled into at two locations almost 100m apart and water circulated between them, passing through the hot rocks. Almost 5 GWh of energy was produced during the test work, using a 60kW binary fluid turbine generator. A key finding of the Fenton Hill and other research in the 1980s was that the enhancement of the permeability of the heat reservoir (done though pumping water into the reservoir at high pressure – a technique from the oil industry) was mainly done via the activation of existing naturally occurring fractures, rather than the artificial creation of a new fracture set.
The work at Fenton Hill to 3,000m depth was followed by Phase II work there, and this research deepened the reach of suitable drilling technologies to beyond 4,000m. By the time it concluded in the early 1990s, the Fenton Hill work showed that drilling (including directional drilling) can achieve depths beyond 4,000m; that this drilling can be controlled and directed in rocks +200°C; that reservoirs can be hydraulically stimulated to produce permeable fracture networks, and that circulation and heat exchange can occur over extended periods to produce electricity

Fenton Hill was followed by the Rosemanowes project in Cornwall and several in continental Europe through the 1980s. In the 1990s drilling commenced at Soultz in France. Several holes here eventually reached 5,000m depth and hydraulic pressurisation again demonstrated that natural fracture networks can be stimulated and enhanced. The Soultz project is on-going.

EGP type geothermal has been embraced enthusiastically in Australia. An early study by Somerville et al (1994) was a “hot dry rock feasibility study” which assessed Australia’s EGP geothermal energy resource and the technical and economic factors involved in hot dry rock energy development. The analysis (which focused on resources in the Great Artesian Basin) showed that the prospects for EGP development in Australia were favourable in terms of the scale of the resource, the efficiency with which the resource could be exploited and the cost of developing the resource.

Geodynamics Ltd have been operating at their ‘Habanero’ project in the Cooper Basin, South Australia for several years and theirs is the most advanced EGP project in Australia. Reported findings of an initial concept study include:

Large scale hot rock geothermal power development may be Australia’s most economic option for generating zero or low emission base-load power;
Well spacing can be increased from the original assumptions of 500m to between 500m and 1,000m with greatly improved economics and project life;
The economic life of an HFR power development is expected to be over 50 years;
Temperature is clearly the most significant economic driver.

Geodynamics drilled their first well to 4,421 m (including 753m in granite with a 6” diameter hole) in 2003. This encountered a fractured, brine saturated system, which was a surprise over the dry rock that was expected. The well was stimulated over several phases and an enhanced fracture system acoustically mapped over 4km². A second well, drilled in 2006 encountered some mechanical problems resulting in the need to drill a side-track hole, which was successful but a dropped plug resulted in the hole not being able to perform the expected test work. However some circulation was achieved between the first and second drill hole . Geodynamics have recently purchased a dedicated deep drilling rig and have announced that the drilling of their third well will occur in 2007.
Although there is no commercial electricity production for an EGP project, on-going work at Habanero and also at Soultz in France indicates that the individual technologies of target selection, deep drilling, reservoir stimulation, sustainable circulation and power generation from hot waters returned to surface should be able to be brought together technically. With appropriate connecting infrastructure and a suitable energy pricing regime, the resultant electricity should find attractive markets.

Direct use of geothermal

Mankind has made use of geothermal energy, expressed through surface warm to hot waters, for thousands of years, including for bathing and heating. Today, conventional drilling technology allows us to tap into suitable aquifers and draw the warm to hot waters to the surface at points where they can be put to domestic or industrial uses. Such uses include building and space heating, air conditioning, drying of agricultural crops, drying in industrial applications and other industrial processes.

Using geothermal energy as a substitute for electricity allows the conservation of electricity and hence the lessening of pollutants associated with the generation of that electricity, such as carbon dioxide.

Low to medium geothermal energy is being used on a modest scale in Australia at the moment, including the heating of swimming pools and the heating of the Geoscience Australia building in Canberra and heating applications in Warnambool, Victoria where hot waters are associated with recent volcanic activity. Again, KUTh believes that the use of such geothermal resources in Australia has not been exploited to the degree possible, due to cheap alternative sources of power, but this situation will not continue due to increasing energy costs and concerns over carbon emissions.